1. Field of the Invention
The invention relates in general to a radio communication system, and more particularly to a method for automatic gain control and antenna selection for a radio communication system.
2. Description of the Related Art
A receiver in a typical radio digital communication system includes at least an antenna, an analog module having radio frequency devices, and a digital signal processing (DSP) module, as shown in FIG. 1. In FIG. 1, a switch 12 switches between two antennas 10 and 11 to select one of them to receive the signal to accomplish antenna diversity. The antenna 10 or 11 receives the signal, which is then processed by the analog module 13 and converted into a digital signal to be transferred to a DSP module 14. The DSP module 14 estimates the power of the digital signal and thus outputs an automatic gain control (AGC) signal to the analog module 13 so as to adjust the gain thereof. On the other hand, the DSP module 14 also outputs a control signal to the switch 12 in order to switch the antenna to receive the signal according to the various conditions.
However, because the power variation range of the received signal of the antenna is quite large, the gain of the analog module 13 has to be properly controlled such that the signal sampled by the ADC (Analog-to-Digital Converter) of the analog module 13 can be properly processed by the DSP module 14. In order to adjust the gain of the analog module 13 in a digital control manner, the conventional way is to adjust the gain according to whether the signal exceeds a specific range of the ADC. In this way, a specific gain is decreased if the gain is greater than an upper limit of the range; and the specific gain is increased if the gain is smaller than a lower limit of the range. This conventional way is similar to the try and error method and has the advantage that it is unnecessary to calculate the signal power. However, the gain cannot be adjusted to the proper range until the adjustments have been tried several times, and it also takes a longer time to reach the stable state. If the antenna selection has to be performed simultaneously, the required time is unavoidably much longer.
However, in some radio communication system, such as the burst mode transmission system, the transmitted packet only has a relatively short preamble signal for the AGC's antenna selection. If the packet specified by the IEEE 802.11a/g is met, the conventional method may encounter some problems. In addition, the gain cannot be adjusted to the proper range in a relatively short time, and there is no time left for the switching to the proper antenna.